384 Sir E. Rutherford and Mr. A. B. Wood on 



are expelled in a very small proportion (1/10000) compared 

 with the ordinary alpha particles. Unless a very intense 

 source be employed, it will not be easy to detect the presence 

 of the long-range alpha particles when the ordinary alpha 

 particles are first absorbed by a layer of 8' 6 cm. of air. 



In the Bragg ionization curves from thorium C given by 

 Marsden and Perkins *, a small residual activity is to be 

 noticed beyond the distance 8'6 cm. which is relatively more 

 marked than for the corresponding curve for radium G. 

 This no doubt is to be ascribed to the effect of the very long- 

 range alpha particles. 



Discussion of Results. 



It is now well established that thorium C is anomalous in 

 breaking up in two distinct ways. One-third of the atoms 

 are transformed with the emission of alpha particles of range 

 5*0 cm., and the remainder gives alpha particles of range 

 8' 6 cm. These modes of transformation of thorium C have 

 been examined in detail by Marsden and Darwin f, and an 

 ingenious scheme of changes has been suggested to account 

 for the facts observed. It is known that the products corre- 

 sponding to thorium G in the radium and actinium series, 

 viz. radium C and actinium C, also have two distinct modes 

 of transformation. Fajans t showed that 1/6000 of the 

 atoms of radium C give rise to a new product of half period 

 1*38 minutes, which emits beta rays in its transformation. 

 In a similar way actinium C has been found to emit two sets 

 of alpha particles of range 5*4 and 6*4 cm.§ This is ascribed 

 to a double mode of transformation, 1*5/1000 of the atoms 

 breaking up with the emission of alpha .particles of range 

 6*4 cm. 



Assuming that the new alpha particles of thorium can be 

 divided into two homogeneous groups of range 10*2 and 

 11*3 cm., it is seen that thorium C must break up in four 

 distinct ways with the expulsion of alpha particles of ranges 

 5-0, 8'6, 10-2, and 11*3 cm. at 15° C. 



The possible modes of transformation of thorium C are 

 thus more complicated than was at first supposed, and it is 

 obvious that the suggestions given by Marsden and Darwin 

 as to the modes of transformation of this substance can be 



* Marsden & Perkins, Phil. Mag. xxvii. p. 691 (April 1914). 



t Marsden & Darwin, Proc. Roy. Soc. A lxxxvii. p. 17 (1912) ; see also 

 Marsden & Barratt, Proc. Phvs. Soc. xxiv. 1, p. 50 (1911); Marsden 

 & Wilson, Phil. Mag. xxvi. p. 354 (1913). 



X Fajans, Phys. Zeit. xii. p. 369 (1911) ; xiii. p. 699 (1912). 



§ Marsden & Perkins, Phil. Mag. xxvii. p. 694 (1914). 



